Time-trend analysis of plasma c-21 steroids in fetal and maternal sheep during the last 18 days of gestation

Daily fetal and maternal plasma samples collected from eight chronically catheterized pregnant ewes from day 130 of gestation through delivery were analyzed for progesterone (P), 20α-dihydroprogesterone [20α-hydroxy-4-pregnene- 3, 20-dione (20P)], 17α-hydroxyprogesterone [17α-hydroxy-4- pregnene-3, 20-dione (17P)], and 11-desoxycortisol [17α, 21-dihydroxy- 4-pregnene-3, 20-dione (Cp S)] by RIAs rigorously validated for specificity in this species. Fetal and maternal P and 20P concentrations fell with advancing gestational age and approaching parturition, while 17P and Cp S concentrations rose. Exponential type regressions for the logarithm of fetal and maternal concentrations of P, 20P, 17P, and Cp S for each animal were plotted as a function of days before parturition. Estimates were made from each fitted curve of the time (T*) for each animal at which changes in P and 20P concentrations began to occur. Mean T* ±SEM values for fetal and maternal P and 20P were: fetal P, −2.7 ± 0.5 days; fetal 20P, -3.0 ± 0.3 days; maternal, −3.5 ± 0.5 days; and maternal 20P, −3.4 ± 0.3 days. Similar time-trend effects suggest a common regulatory focus for P and 20P production. P concentrations were markedly higher in maternal than fetal circulation, while the reverse was found for 20P. This finding is consistent with a highly active fetal 20α-reductase system. Analysis of 17P produced exponential-type curves of nonuniform shape, strikingly different from P and 20P. These curves indicate different time-trend effects and imply that other regulatory foci are involved in 17P production. Exponential-type curves for Cp S in both fetal and maternal circulations were either concave upward or positive sloping straight lines, a finding which may reflect the adrenal origins of this steroid.